Paragon CRT Dual Axis Quick Reference Manual

Brand: Paragon

Model: CRT Dual Axis

Type: Quick Reference Manual

Paragon CRT Dual Axis

Quick Reference Guide

DUAL

AXIS

CRT

PARAGON CRT DUAL AXIS

OPTIONS TO COMPENSATE

FOR PERIPHERAL CORNEAL ELEVATION DIFFERENCES

Most eyes with corneal astigmatism manifest a signicant dierence in

elevation between the steep and the at corneal meridians (Image A).

Some corneas that appear spherical may also manifest a peripheral corneal

elevation dierence. e Paragon CRT® lens design has a xed chord length

of 8 mm (6 mm treatment zone surrounded by a 1 mm return zone width).

When the elevation dierence exceeds approximately 30 microns at a chord

diameter of 8 mm, it may not be possible for the lens to reach the cornea

circumferentially in the landing zone (Image B). is creates a weakened

compressive force in the steep meridian and results in poor centration,

lens exure, and/or undertreatment. An example of an elevation map with

elevation values at 4 mm from reference center in each meridian is shown

in Image C.

A common rule in contact lens tting holds that spherical lenses will always

move freely along the steep meridian and exhibit restricted movement

along the at meridian. e clinician will observe that spherical lenses will

touch rst in the attest meridian peripherally and rock, ex or tilt over the

steepest meridian. In conventional corneal reshaping designs, the lens can

only reach the peripheral steep meridian by compressing or applanating the

at meridian or by way of lens exure. Oval treatment zones are commonly

observed with peripheral corneal elevation dierences.

Aptical Astigmatism

Image A Image B Image C

Limbus to Limbus

Astigmatism

8mm Chord Values

e Paragon CRT® Proximity Control Technology™ and manufacturing

system was designed to allow for independent modulation of the three zones

of the lens: Base Curve (BC) Return Zone Depth (RZD), and Landing Zone

Angle (LZA). e Base Curve radius controls treatment of the refractive

error and remains spherical and unchanged. In the Paragon CRT Dual Axis®

version, the Return Zone Depth (RZD) can be varied in selected meridians

to compensate for the peripheral corneal elevation dierences of an eye.

Correspondingly, the Landing Zone Angle, by itself, may be varied in a

second axis while the base curve and return zone depths are unchanged or

both RZD and LZA may be varied in the second meridian to compensate for

the peripheral corneal slope meridional dierences of an eye.

In other words, when tting a Paragon CRT Dual Axis® design, the BC

remains constant throughout the treatment process, unless there is a need

to correct untreated or residual power.

Using this standard CRT lens example: BC 8.7 RZD 550 LZA -33

e possible parameter alternatives for the Dual Axis product are:

1. e RZD could have a second value and the LZA

would remain a single parameter (8.7/550 & 600/-33)

2. e LZA could have a second value and the RZD

remains a single parameter (8.7/550/-33 & -34)

3. e RZD and LZA could both have second values (8.7/550 & 600/-33 & -34)

Corneal Topography and CRT Dual Axis

Prescribing

It is common for Placido-based corneal topographers to directly measure curvature

data out to about 9-10 mm. is curvature data can be reconstructed into height data

and represented as an ELEVATION MAP. Elevation maps illustrate relative height

deviations from the best t reference sphere for a particular cornea in MICRONS.

On a cornea with asymmetric peripheral saggital depth, a relative deviation from a

reference sphere (RED shaded areas) can be observed between the meridian having

the greatest elevation above the reference sphere and the meridian having the lowest

elevation below the reference sphere (BLUE shaded areas). By placing the cursor

at a point 4 mm from the center of the plot, the local elevation may be observed

along the CRT lens chord length. If the total dierence is greater than 13 microns

on the two meridians at the 8 mm chord, a Paragon CRT Dual Axis® option may be

indicated.

When to consider Paragon CRT

Paragon CRT Dual Axis

Dierence In Flat & Steep Meridians at 8 mm chord Suggest Using is design

<30 Microns Paragon CRT®

>30 Microns Paragon CRT Dual Axis®

Calculating for CRT Dual Axis

from Elevation Dierences

1. Select the elevation plot feature on the corneal topographer. Look at the elevation

plot and observe the presence of signicant elevation dierence (BLUE vs. RED) in

the periphery of the plot.

2. Place the cursor on the shallow area (BLUE in color), 4 mm from the center of the

plot and record the number of microns below the reference sphere.

3. Repeat the measure 180 degrees away in the shallow area opposite to the rst

measurement.

4. Place the cursor on the elevated area (RED in color), 4 mm from the center of the

plot and approximately 90 degrees from the shallow area and record the number of

microns above the reference sphere.

5. Repeat the measure 180 degrees away in the elevated area opposite the rst

elevated measurement.

6. Calculate the average dierence from the elevated areas to the shallow areas.

7. If the average dierence is:

• less than 30 microns, a Paragon CRT Dual Axis® lens may not be indicated.

• between 25 to 30 microns, consider prescribing a one-step (25-micron)

increase in RZD.

• between 31 to 60 microns, consider prescribing a two-step (50-micron) RZD

dierence.

• between 60 to 90 microns, consider prescribing a three-step (75-micron)

RZD dierence.

• > 90 microns, consider prescribing a four-step (100-micron) RZD dierence.

e average elevation dierences are calculated using this best t sphere example:

A standard two 25-micron step (50-micron) dierence in the RZD, 90 degrees

apart is oered to compensate for dierences in peripheral corneal elevation. e

recommended method of tting is to continue the use of the RZD suggested by the

Initial Lens Selector (ex: 550) and observed to provide the recommended uorescein

pattern. (Image C) is RZD continues to be recommended for the at meridian

with the respective higher peripheral elevation. An RZD increase of 50 microns

(two 25-micron steps) is recommended for the steep meridian with the lower or

shallower peripheral elevation. (Image D) In this case, no change is recommended

in the LZA. e LZA will remain constant circumferentially. Some eyes may

Image A Image B

Image C

manifest more than 50 microns of meridional dierence in elevation at the 8 mm

chord diameter. In such cases, a three-step, 75-micron or four-step, 100-micron

meridional dierence in the RZD may be indicated.

When tting Paragon CRT Dual Axis®, use this chart to determine the variation of

RZD or LZA in conjunction with the average elevation dierences.

When to Change the LZA?

One may observe a cornea that does not manifest a meridional elevation dierence

at the 8mm chord diameter but does manifest signicant dierences in the edge

clearance circumferentially. In such cases, the Landing Zone Angle should be varied

in a second axis to provide a more uniform circumferential edge clearance. e

recommended method of tting is to continue to use the LZA that appears to be

correct in the meridian having proper edge clearance and increase the LZA for

the alternate meridian where there is excessive edge clearance. A dierence of one

degree in the second meridian will be clinically signicant and a dierence of two

degrees may be needed when there is a marked dierence in the edge clearance

observed in the uorescein pattern.

If the Average

Meridional

Elevation

diﬀerence is

Use This RZD in

the Elevated/Red

Meridian

RZD suggested by

CRT Lens Selector

(550)

Increased the RZD

used in Flat

Meridian by 50

Microns (550 & 600)

LZA suggested by

CRT Lens Selector

(-33)

Add one degree if

cdgc lift is exessive

(-33 & -34)

Add one degree if

cdgc lift is exessive

(-33 & -34)

Add one degree if

cdgc lift is exessive

(-33 & -34)

LZA suggested by

CRT Lens Selector

(-33)

LZA suggested by

CRT Lens Selector

(-33)

Increased the RZD

used in Flat

Meridian by 50

Microns (550 & 625)

Increased the RZD

used in Flat

Meridian by 50

Microns (550 & 650)

RZD suggested by

CRT Lens Selector

(550)

RZD suggested by

CRT Lens Selector

(550)

30 - 60 Microns

60 - 90 Microns

> 90 Microns

Use This RZD in

the Shallow/BLUE

Meridian

Use This LZD in

the Elevated/Red

Meridian

Use This LZD in

the Shallow/BLUE

Meridian

Image D

Using the 16-lens trial set of CRT Dual Axis

lenses,

follow these procedures:

1. Aer evaluating the suggested spherical lens from the Paragon CRT® Initial Lens

Selector (example: 8.60/550/-33), place the dual axis lens having the nearest BCR,

but same RZD, and LZA. e RZD for the deeper meridian will be 50 microns

greater (8.90/550 & 600/-33). A RZD with a 75 micron dierence would be for a

cornea with more corneal astigmatism than normal.

2. Observe the lens on eye for the criteria for an ideal t:

a. A minimum of 4 mm treatment zone that is round in appearance

b. Lens centered within 0.5 mm of pupil center

c. Circumferential bearing under the landing zone (complete/uniform “bulls eye”

pattern)

d. Edge clearance between 0.2 and 0.5 mm

3. If the edge clearance is signicantly dierent 90 degrees apart, consider a dual axis

LZA (33 & 34) dierence where the angle is increased by one degree for every 0.2

mm required decrease in the edge clearance in the respective meridian (8.90/550 &

600/-33 & -34).

4. Over-refract for the appropriate BCR and order the nal Paragon CRT Dual Axis®

lens. It is of critical importance that the desired meridian is identied relative to the

shallow and deep RZD meridians. In some cases you may need to deepen the LZA

in the same meridian that is already made deeper by the RZD increase.

Prescribing Paragon CRT Dual Axis

Lenses in the

Absence of Topographic Elevation Map Options

e Dual Axis feature can be successfully used in the absence of elevation data by

careful identication of corneal astigmatism on the AXIAL topography map as well

as careful observation of the uorescein pattern of the initial Paragon CRT® lens

parameter suggested from the at K and spherical refraction. e following

are the indicators for a dual axis design:

• Limbus-to-limbus corneal astigmatism greater than 1.00 D

(pre treatment axial map)

• Oval versus the round treatment zone observation in a best-t Paragon CRT®

spherical diagnostic lens

• Signicant dierence in edge clearance 90 degrees apart with a best-t

spherical Paragon CRT® diagnostic lens

• Flexure as measured by keratometry or topography over a best-t Paragon

CRT® lens

• Failure to center or complete treatment with a spherical Paragon CRT® lens

using suggested parameter changes

• Decentered treatment zone on follow up with an otherwise best-t Paragon

CRT® lens

Dierences of 30 microns are sucient to cause exure, decentration, and

undertreatment due to weakened circumferential attraction between the landing

zone of the lens and the peripheral cornea. Consider use of the two RZD dierence

(50-micron) design when these indicators are present.

e indication of the Paragon CRT Dual Axis® design is conrmed if the treatment

zone becomes more circular, exure is eliminated, lens centration is improved,

circumferential landing is observed, and the targeted treatment is achieved.

For additional information on Paragon CRT Dual Axis

please call your Authorized CRT Laboratory Consultant,

or Paragon Consultation at: 800-528-8279 option 2.

ZQF100013E - 11/17

1. Anderson D. Map out Your Lens Fitting. Optom Manag 2008 Oct; 43(10): 26-30

2. Anderson D, Kojima R. Understanding and Applying Corneal Topography. Rev Cornea & Contact Lenses 2007 Sept: 22-27

3. Caroline P, Andre M. Not All Astigmatism is Created Equal. CL Spectrum 2009 Apr: 24(4):56

4. Caroline P, Andre M. Elevating Our Knowledge of the Corneal Surface. CL Spectrum 2001 Apr; 16(4):56

5. Roberts C. A Practical Guide to the Interpretation of Corneal Topography. CL Spectrum 1998 Mar: 13(3):25-33

DUAL

AXIS

CRT

Paragon CRT Dual Axis Quick Reference Manual

Paragon CRT Dual Axis Quick Reference Manual

Paragon CRT Dual Axis Quick Reference Manual

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